Aeroponic growth systems provide many advantages for agricultural research and production. The plant tissue is placed in a growth chamber and is moistened with a nutrient fog. For seedlings, the roots are in a separate part of the chamber from the foliage. The roots are maintained in a dark, nutrient fog-filled portion of the growth chamber and the foliage is maintained in an illuminated portion of the chamber. For optimum growth, control of the nutrient fog density and composition is required.
Most commonly the nutrient fog is generated by misting nozzles in the growth chamber, which are turned on at timed intervals. There are several disadvantages to this method of fog production. Surplus nutrient solution collects at the bottom of the growth chamber and is either discarded, thereby requiring greater nutrient use, or recycled, at the risk of contamination. The fog production is intermittent so the operating conditions cannot be stable. Because of the droplet size the roots can become coated with a water layer which interferes with nutrient absorption. Accumulation of nutrient salts on the nozzles requires frequent cleaning and maintenance.
Ultrasonic fog generation overcomes many problems of spray-based fog generation. An ultrasonic transducer is positioned in the bottom of a nutrient solution reservoir and a nutrient fog forms above the liquid. The reservoir can either be in the bottom of the growth chamber or the reservoir can be separate from the growth chamber and the fog can be carried into the growth chamber by a stream of air. Little surplus nutrient solution collects in the growth chamber, the ultrasonic transducer can be operated continuously, and the droplets are small enough that a water layer does not form on the roots.
Nonetheless there are several factors which prevent stable ultrasonic fog generation. Recognition of these factors is one aspect of the present invention. The humidity level of ultrasonically produced fog is 100%. When fresh air is passed over the nutrient solution reservoir, the humidity level of the air determines how much of the nutrient solution is vaporized to increase the humidity back to 100% and how much is atomized into fog. Changes in the humidity and temperature of the intake air change the nutrient concentration in the fog. Additionally, as the nutrient solution reservoir is depleted, due to vaporization and atomization of the nutrient solution combined with resultant precipitation of nutrients from the fog, the salt concentration in the nutrient solution reservoir increases significantly. The fog can become so concentrated that it is toxic to plants.